In the civil aviation domain, it is known to use an additional thermal exchanger to cool the oil that circulates in the turbojet engine. Hot oil is brought in the thermal exchanger to be cooled before being reused in the propulsive system.
In cross-section in prior art FIG. 1 of the state of the technique, a turbojet 1 is represented as well as two heat exchangers 6 and 12 representing the state of the technique.
Turbojet 1 includes a nacelle 2 in which is placed an engine 3. Engine 3 is attached to internal wall 4 of nacelle 2 through air bifurcations 5.
In the state of the technique, there are generally two possible positions for the thermal exchanger 6 and 12 The thermal exchanger 6 and 12 can be positioned integral with the engine body 3, or integral with the nacelle 2.
When thermal exchanger 6 is mounted at the level of engine body 3, it is more precisely fitted in an internal volume 7 provided between a motor cover 8 surrounding at least partially engine 3, and engine 3 itself. An air intake 9 takes cold air in the cold air flow passing through turbojet 1, to bring it inside thermal exchanger 6. Cold air passes through the thermal exchanger matrix, in which circulates the hot oil to be cooled. Both fluids are separated one from the other by baffles, and don't mix. Heat exchange takes place inside the matrix. Partially reheated air exit from thermal exchanger 6, through air outlet 10, to be reinjected in the secondary air flow 14 exiting from the nacelle.
In case thermal exchanger 12 is positioned at the level of nacelle 2, it is more precisely fitted in the internal volume of said nacelle 2. An air intake 13 takes cold air in the cold air flow passing through turbojet 1, to bring it inside said thermal exchanger 12. After passing through the matrix of thermal exchanger 12, this air flow is either ejected outside nacelle 2 through air outlet 14, or reintroduced in the internal flow of the engine through a specific air outlet (not represented).
Such thermal exchangers 6 and 12 don't prove to be an optimal solution in term of propulsion efficacy and aerodynamic impact on the engine, and this for several reasons. In case the air that crosses the exchanger matrix is rejected outside of the internal flow of the engine, i.e. in case the heat exchanger 12 is installed with air outlet towards the outside as depicted in FIG. 1, the air intake 13 constitutes a direct loss of propulsive efficacy insofar as it contributes little, if at all, to the engine thrust. In case the air that passes through the thermal exchanger matrix is reintroduced in the internal flow of the engine, as is the case of an installation of heat exchanger 6 inside the engine body 3, the thermal exchanger matrix, from its internal architecture, induces a large load loss in the flow and the flow through air output 10 tends to disrupt more or less significantly the downstream aerodynamic flow of the engine. In addition, the presence of an air intake 9, with one or several internal ducts, as well as an air outlet 10 generates load losses and disrupts more or less significantly the internal flow of the engine 3.
Another known solution is to use a plate exchanger 15. A plate exchanger 15 is notably known to match locally the shape of internal wall 4 of nacelle 2 to which it is coupled. Lower face 16 of the thermal exchanger is coupled to internal wall 4 of the nacelle, while a upper face 17 is located in the cold air flow that passes through the internal volume of nacelle 2. The heat transported within the exchanger 15 is transferred by thermal conduction to the internal surface of the plate forming the upper face 17 of aforesaid thermal exchanger. This hot plate is cooled by the passage of cold secondary air 14 flowing into nacelle 2. The heat stored in the hot plate is thus dissipated by forced convection toward the aerodynamic flow of turbojet 1.
One inconvenience of the plate exchanger 15 embodiment of a thermal exchanger of the state of the technique is that it is incompatible with the current systems for reducing sound nuisance coming out of the turbojet. Indeed, in order to reduce said sound nuisances, it is known to at least partially cover internal wall 4 of nacelle 2 with an acoustic coating 11. More generally, such acoustic coating 11 covers the internal and external walls of nacelle 2 and motor cover 8 since these walls are facing each other. The presence of such acoustic coating 11 is incompatible with the coupling of the plate thermal exchanger on internal wall 4 of nacelle 2. It would require, in order to use such plate thermal exchanger, to suppress acoustic coating 11 locally, which proves difficult given the dimensionality specifications related to sound nuisance.